FIG. 1. illustrates a prior-art Multiple-Input-Multiple-Output (MIMO) transmission scheme implemented in an Orthogonal Frequency Division Multiplexing (OFDM) system. In the system of FIG. 1, transmitting device 102 employs multiple antennas 104 to transmit multiple data streams across multiple OFDM subcarriers to one or more receiving devices 101. The multiple data streams are denoted si(k), where the index i denotes the stream number (1≦i≦Ns) and the index k denotes the subcarrier (1≦k≦N), where N is the number of subcarriers, Ns≧1 is the number of data streams per subcarrier. The signal fed to each of the Mtx transmit antennas 104 is generated by an Inverse Fast Fourier Transform (IFFT) 108, a Cyclic Prefix Insertion Device 107 and a Parallel to Serial Converter 106. The OFDM transmission technique divides up the occupied frequency bandwidth into N orthogonal subcarriers, where each input to the IFFT corresponds to a subcarrier, and the signal fed into each input of the IFFT is said to occupy the corresponding subcarrier. The N inputs to each IFFT are called subcarriers, and in prior-art single transmit antenna OFDM systems, a coded modulation (i.e., QAM or PSK) symbol would typically be fed into the subcarrier inputs of the IFFT, one symbol per subcarrier, or equivalently, one data symbol stream per subcarrier. However, in the prior art MIMO-OFDM system shown in FIG. 1, on a given subcarrier (say the kth), the Ns symbols for the multiple streams are instead first fed into a multi-stream transmit beamformer 105 having Ns inputs and Mtx outputs (where Mtx is the number of transmit antennas). The Mtx outputs of each beamformer 105, denoted xm(k) (1≦m≦Mtx, 1≦k≦N), are then fed to their respective subcarrier inputs on the Mtx IFFTs 108. In one embodiment of the prior-art MIMO-OFDM transmitting device 102, the number of streams Ns is equal to the number of transmit antennas Mtx, and on each subcarrier (say the kth), the i−th data stream for the kth subcarrier is fed to the kth subcarrier input of the ith transmit antenna, and the multi-stream transmit beamformers 105 are not used. However, better performance can often be obtained if the beamformers 105 are used prior to the IFFT subcarrier inputs.
FIG. 2 illustrates in more detail a prior-art multi-stream transmit beamformer 105 that is used prior to each subcarrier input on the Mtx IFFTs 108 of the transmitting device 102. As shown in FIG. 2, each multi-stream transmit beamformer 105 of the transmitting device 102 employs transmit antenna array weights denoted with the Mtx×Ns matrix V(k)=(v1(k), v2(k), . . . , vNs(k)), where Mtx is the number of transmit antennas, Ns is the number of data streams being delivered on the kth subcarrier, and the ith column of V(k) is denoted by the Mtx×1 column vector vi(k) that contains the Mtx weighting coefficients for the ith data stream on the kth subcarrier. In order to calculate appropriate values for V(k), the transmitting device 102 generally requires some information about the channel response between transmitting antennas 104 and the one or more receive antennas on the receiving devices 101.
Returning to FIG. 1, receiving device 101 measures the downlink channel response and is responsible for sending back the information that is to be used by transmitting device 102 to compute the transmit antenna array weights (V(k)=(v1(k), v2(k), . . . , vNs(k)), where k is the subcarrier and Ns is the number of data streams per subcarier) that are applied to each subcarrier data stream. Typically, this information being sent back consists of the transmit weight vectors or an index into a codebook of possible weight vectors, or other information based on a similar technique. The Ns data streams are then multiplied by weight vectors V(k=1) through V(k=N) in order to effectively deliver the multiple streams to the receiver 101.
As is evident, such a system requires transmitter 102 to know the matrix frequency response between the transmit and receive arrays, which can pose difficulties in rapidly-varying frequency-selective broadband channels, such as the type of channels encountered in mobile communication systems employing OFDM. More particularly, in a feedback-based transmit array system, such as that illustrated in FIG. 1, the transmit weight vector needs to be fed back to transmitter 102 and updated to track the channel variations that occur across time and frequency for optimal performance. Unfortunately, a mechanism that allows the complete tracking of the channel response may require prohibitive levels of feedback between the receiving device and the transmitting device. Therefore, a need exists for a method and apparatus for multi-antenna transmission within a communication system that does not require prohibitive levels of feedback when sending back channel information to be used by the transmitting device.